Fire retardant wood and method for the production thereof



United States Patent 3,537,887 FIRE RETARDANT WOOD AND METHOD FOR THE PRODUCTION THEREOF Clyde L. Blohm, Burbank, Calif., assignor to Flame-X- Control Corporation, Los Angeles, Calif., a corporation of California No Drawing. Filed Mar. 27, 1967, Ser. No. 625,953 Int. Cl. B44d 1/26 US. Cl. 117-116 9 Claims ABSTRACT OF THE DISCLOSURE A weather resistant, fire retardant wood having substantially the same impact strength as the original wood, said wood having a portion which has distributed therethrough the reaction product of urea-phosphoric acid and cellulose, the cellulose being part of the wood. A method of producing such a fire retardant wood by impregnating wood in an aqueous solution of urea, phosphoric acid, and a volatile base at a relatively high pressure. The mole ratio of urea to phosphoric acid is from 2 6:1 and the pH of the aqueous solution is between and 9. The impregnated wood is then placed in an enclosed container containing a water-immiscible organic liquid and the water contained in the wood and the organic liquid are distilled off. The preferable volatile base is ammonia while the preferred water-immiscible organic liquid is perchloroethylene.

BACKGROUND OF INVENTION The present invention relates to a method, and to the composition produced therefrom, for rendering wood, and in particular wood shingles or shakes, fire retardant.

Among other discoveries made by men only a few have borne as many fruits as has the discovery of how to make and use the type of combustion commonly known as fire. The use of fire is particularly indispensible to modern civilization; it is a basis of almost all forms of manufacturing and transportation. In spite of all the benefits derived from fire, the mere word Fire! is one of the most feared expressions in any language for fire is almost as dangerous as it is useful.

As civilization becomes more and more complex and as people tend to live closer and closer together the prob lem of preventing or controlling accidental fires, particularly in dwelling houses, has greatly increased from the time man only had to worry about his own particular dwelling house. Among the most vulnerable portions of the house, insofar as fire is concerned, is the roof. When any house or tree catches on fire, generally the flames and glowing or flaming brands are carried by the wind to the roofs of any nearby houses. The vulnerability of roofs is so acute in certain areas that insurance companies will not insure houses against fire loss unless the roof has been rendered fire-retardant in accordance with the criteria set forth in certain Underwriters tests or government regulations.

For the most part, roofs have been fire-proofed by using slate shingles or the like. Although these roofs are relatively satisfactory for purposes of fire-proofing they suffer the disadvantage of being relatively expensive and, in addition, necessitate the use of a stronger house frame which also adds to the expense. It is therefore not surprising that there have been many attempts to produce a satisfactory fire retardant wood shingle. These attempts, for the most part, have failed notably.

In view of the superficial relationship between certain types of cellulose textile fabrics and wood, those unskilled in the art may think that a process which is useful in im- "Ice parting fire resistance to cellulosic textile fabrics would also be useful in the flame retardation of wood. This, however, is not true due to certain basic differences in the two types of materials.

US. Pat. 2,482,755 discloses a process for imparting fire resistance to cellulosic and protein fibrous textiles by impregnating said textiles with a mixture of urea and phOsphoric acid. After the aforesaid mixture is impregnated in the textile it is cured at about 140 C. in order to form a complex polymer. According to US. Pat. 2,917,408, this process, when applied to wood, has a very deleterious effect on the wood in that, inter alia, the impact strength of the wood is materially reduced which, for all intents and purposes, makes the wood unsatisfactory for use as shingles.

According to Pat. 2,917,408, when the curing temperature was lowered sufiiciently to avoid degradation of the wood, the temperatures were not sufficient to permit the reaction between the mixture of urea and phosphoric acid and the wood and, therefore, the mixture was easily leached out of the wood thereby rendering the wood no longer fire retardant. US. Pat. 2,917,408 attempts to solve the problem of producing a fire retardant wood which will withstand leaching and not lower the impact strength of the wood by impregnating the wood with a. solution of dicyandiamide and phosphoric acid and curing the impregnated wood by heating to a temperature of from 70 to C.

SUMMARY OF THE INVENTION The principal object of the present invention is to disclose and provide a method for producing a fire retardant wood which will withstand Weathering without reducing its fire retardant characteristics.

A further object is to disclose and provide a method for producing a fire retardant wood which does not substantially decrease the strength of the original wood.

Another object is to disclose and provide a fire retardant wood which has good impact strength and weathering characteristics.

Generally stated, the present invention is based upon the surprising discovery that a fire retardant wood, as described above, can be produced by immersing the wood in an aqueous solution of urea, phosphoric acid, and a volatile base at a relatively high pressure, e.g., above 30 p.s.i. and preferably from 50 to 200 p.s.i.

During immersion, the vessel containing the wood and the aqueous solution of urea, phosphoric acid, and volatile base can be either evacuated or not, but the pressure must be maintained for at least about two hours, preferably at least 6 to 8 hours (there is no upper maximum) or until the wood has taken up at least 25% (preferably 50%-100%) of its weight of the aqueous solution. Thereafter, the wood is taken out of the solution and placed into a distillation kettle or any other suitable vessel and a water-immiscible organic liquid having a boiling point, at atmospheric pressure, between about and C. is added. The wood and organic liquid are then heated to a temperature sufficient to vaporize the organic liquid; the amount of the solvent being sufiicient to completely encompass the wood when the solvent is in the vapor phase. This inter alia, prevents air from being in contact with the wood during the heating and curing step, which is critical in order to avoid reducing, to any great extent, the impact strength of the wood.

During this boiling or heating step the urea-phosphoric acid and the cellulose in the wood are polymerized, certain reaction products are formed and water is released from the wood. The reaction products and the water are allowed to escape and the organic liquid and wood are maintained at the aforesaid temperature until the polym- 3 erization reactions are completed, usually at least two hours and preferably at least eight to ten hours.

The water released from the wood during the heating step forms an azeotropic mixture with the organic liquid which has a lower boiling point than the organic liquid. The heating or boiling step can be discontinued after the boiling point has risen to that of the organic liquid, which indicates that water is no longer being released from the wood. In practice, however, this end temperature is maintained for from one to several hours in order to insure completion of the polymerization reactions.

It should be noted that the above heating step is conducted at a temperature which U.S. Pat. 2,917,408 states destroys the impact strength of the wood. Surprisingly, there is minimum loss of impact strength of the wood when using the process of the present invention. This surprising result is in direct contrast to the teachings contained in the prior art.

Preferably, the aqueous solution referred to above, contains from 20 to 50% by weight of the mixture of urea, phosphoric acid, and volatile base. The mole ratio of urea to phosphoric acid is preferably about 26: 1. The amount of volatile base added is sufficient to raise the pH of the final aqueous solution to from about 5 to 9. The preferred base is ammonia.

Although it is not necessary, one may add a lower molecular weight aliphatic aldehyde having from one to four carbon atoms such as formaldehyde or glyoxal to the aqueous solution with no deleterious results. The amount of formaldehyde or glyoxal is not critical but best results have been obtained by using about one mole of formaldehyde or glyoxal to one mole of urea. However, it should be particularly emphasized, that no deleterious results are obtained when using more or less of the formaldehyde or glyoxal, e.g. from 0.5 to 3 moles of formaldehyde or glyoxal per 1 mole of urea.

The fire retardant wood produced in accordance with this invention has a phase which consists of the reaction product of urea-phosphoric acid and the cellulose of the wood. The immersion process wherein the wood is immersed in the aqueous solution under a relatively high pressure allows the aqueous solution to penetrate deeply into the wood; if the wood is sufl'iciently thin the aqueous solution will penetrate to substantially the center. Thus, when the wood is heated in the distillation kettle, the phosphoric acid-urea reaction product will chemically combine with the cellulose in the wood and the wood and the reaction product will be integrally bound, which, because of the deep penetration of the aqueous solution, will prevent the wood from losing its fire retardant properties when exposed to the effects of weather.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Immersion process Two bundles of heavy wood shakes were placed in a pressure autoclave and completely immersed in a ureaphosphoric acid solution having the following composition: 160 lbs. of urea, 120 lbs. of 75% phosphoric acid, 86 lbs. of a volatile base (aqua ammonia, a solution of 28-29% NH in water), and 730 lbs. of water. This solution has approximately a 25% concentration of the mixture of urea, phosphoric acid and ammonia.

The pressure was raised in the autoclave to 150 psi. and the two bundles of shakes were left immersed in the aqueous solution at this pressure for 6 hours. After this time the pressure was released and the two bundles of shakes were removed and allowed to drain for minutes. One bundle had picked up 37 lbs. of solution while the other bundle had picked up 49 lbs. of solution. The bundles initially weighed 51 lbs. and 56 lbs., respectively. The bundles were then air dried for several days.

As has already been noted, the ratio of the various components of the aqueous solution is not critical provided that the mole ratio of urea to phosphoric acid is 26: 1. Similarly, the amount of volatile base is not critical provided that the aqueous solution has a pH of from 5 to 9; the amount of base to produce such a pH varies with the particular volatile base used.

By volatile base, is meant any organic base which has a boiling point of less than about 150 C. and preferably less than C. Preferred volatile organic bases are lower alkylamines and diamines having a boiling point below C. Examples of such lower alkylamines and diamines are: (1) diethyl amine, (2) dipropyl amine, (3) ethyl amine, (4) methyl amine, and (5) ethylene diamine.

If desired, up to lbs. of formaldehyde may be added to the above aqueous solution of urea, phosphoric acid, and ammonia.

As is apparent from the gain in weight of the bundles of shake (in one case the gain in weight was almost 90% the aqueous solution penetrated deeply into the center of the wood. This phenomenon partially accounts for the fact that the wood shingles or shakes produced in accordance with the method of this invention are able to withstand weathering without loss of any fire retardant property.

Curing process The two bundles of air dried shakes produced in accordance with the process above were placed in a distillation still, the still was filled with a water-immiscible organic liquid (perchloroethylene) and the still heated by an external oil bath. The first distillate distilled over at the boiling point of the azeotrophic mixture of perchloroethylene and water, approximately 88 C. Distillation was continued for a total of about 12 hours during which time the temperature of the distillation gradually increased to 121 C., the boiling point of perchloroethylene at atmospheric pressure. After this temperature was obtained heating continued for an additional three hours at which time the heating was discontinued and the perchloroethylene was drained from the still. The still was opened and the bundles removed. The individual shakes were fire retardant and resisted leaching.

Instead of using perchloroethylene any water immiscible organic liquid having a boiling point, at atmospheric pressure, of between about 120 and C. can be used. The following list of water immiscible organic liquids are particularly preferred and can be used with equally as good results as were obtained with the use of perchloroethylene.

Water immiscible organic liquid: Boiling point, C.

(1) Any nonane isomer or mixture When using the above organic liquids the same procedure is used as when perchloroethylene is used. The heating is continued for a period of at least about 6 hours, and up to 12 hours. The length of the heating step, of course, depends upon the boiling point of the particular water immiscible organic liquid which is being used; the higher the temperature at which the water immiscible organic liquid boils the shorter the heating time which is necessary. The most desirable heating time can easily be determined by routine experiments.

The fire retardant shakes produced in accordance with the foregoing preferred exemplary embodiments have a phase, extending from the surface to deep Within the wood, consisting of the reaction product of urea-phosphoric acid and wood cellulose. The phosphoric acid and urea constitute from to by weight of the shake or shingle.

I claim:

1. A method for producing a weather resistant, fire retardant wood without greatly impairing the impact strength of the wood comprising:

(1) impregnating the Wood for at least about two hours under a pressure of at least p.s.i. in an aqueous solution of (a) urea, (b) phosphoric acid, (c) a volatile base; the mole ratio of urea to phosphoric acid being from 26:1 and the amount of volatile base being sufiicient to raise the pH of the aqueous solution to between 5 and 9,

(2) removing the wood from the aqueous solution,

(3) placing the wood in an enclosed container containing a sufficient amount of a water-immiscible organic liquid to completely encompass the Wood when said organic liquid is in the vapor phase, said water-immiscible organic liquid having a boiling point, at atmospheric pressure, of from 120 to 175 C. and,

(4) heating the wood and organic liquid to about a temperature sufficient to vaporize said organic liquid and maintaining said organic liquid in the vapor phase at said temperature until the polymerization reaction is completed and allowing the reaction prod ucts to distill off.

2. A method according to claim 1 wherein the pressure at which the wood is impregnated is from to 200 ps1.

3. A method according to claim 1 wherein the base is ammonia.

4. A method according to claim 1 wherein the waterimmiscible organic liquid is perchloroethylene.

5. A method according to claim 1 wherein the wood is impregnated with said aqueous soluton of urea, phosphoric acid and a volatile base until the wood has taken up at least 25% of its weight of said aqueous solution.

6. A method according to claim 1 wherein the mixture of urea, phosphoric acid and volatile base constitutes from 20% to 50% by weight of the aqueous solution.

7. A method for producing a weather resistant, fire retardant wood without substantially impairing the impact strength of the Wood comprising:

(1) impregnating the wood for at least 6 hours under a pressure of from 50 to 200 p.s.i. in an aqueous solution containing from 20% to 50% by Weight of a mixture of (a) urea, (b) phosphoric acid, and (c) a volatile base; the mole ratio of urea to phosphoric acid being from 26:1 and the amount of ammonia present being sufficient to raise the pH of the aqueous solution to between 5 and 9,

(2) removing the wood from said aqueous solution,

(3) placing the wood in an enclosed container containing sufiicient amount of a water-immiscible organic liquid to completely encompass the wood when said liquid is in the vapor phase, said water-immiscible organic liquid having a boiling point, at atmospheric pressure, of from to C.

(4) heating said mixture of wood and organic liquid to the temperature sufficient to vaporize said organic liquid and maintaining the organic liquid in the vapor phase for at least 8 hours.

8. A method according to claim 7 wherein the waterimmiscible organic solvent is perchloroethylene.

9. A method according to claim 7 wherein the volatile base is ammonia.

References Cited UNITED STATES PATENTS 2,482,755 9/1949 Ford et al 117-137 X 2,860,070 11/1958 McDonald 117-116 X 3,242,004- 3/1966 Kenaga 117-148 X 3,253,881 5/1966 Donahue 117137 X RALPH S. KENDALL, Primary Examiner US. 01. X.R:.) g1 7; 117-137, 1425 

